A microwave multiplexing network is used to combine or separate microwave frequency bands (i.e. those that exist in the range of 100 MHz to 100 GHz) and typically consists of a plurality of channel filters operatively coupled to an interconnect such as a waveguide manifold. Usually, the channel filters are sequentially arranged along the waveguide manifold according to center frequencies with the highest frequency channel or with the lowest frequency channel positioned adjacent to the shorting plate of the waveguide manifold. However, non-sequential arrangement is also feasible.
Channel filters are devices that are tuned to pass energy in a desired frequency range (i.e. the passband) and to reject energy at unwanted frequencies (i.e. the stopband). Channel filters are also designed to meet various performance criteria such as a particular level of insertion loss (IL), which is also known as rejection or isolation, and return loss (RL). The order of the channel filter is equivalent to the number of poles in the transfer function and the higher the order the more rejection a channel filter can provide. The number of poles can be seen by looking at a graph of the return loss wherein each peak represents one pole in the transfer function. For each pole there is a physical electrical cavity present in the channel filter. For example, a four-pole filter will have four electrical cavities and a five-pole filter will have five electrical cavities.
As conventionally known, a higher order filter provides greater rejection (i.e. insertion loss) than that of a lower order filter. Accordingly, the use of a high order filter allows for the bandwidth of the channel filter to be expanded since the extra pole(s) provide extra rejection. Overall this results in increased filter bandwidth. At the same time, reasonable filter rejection is maintained. For example, a five-pole filter provides a larger filter bandwidth than that of a four-pole filter because the fifth pole provides extra rejection that allows for the widening of the passband of each channel filter. While the overall filter rejection level associated with the five-pole filter will be reduced due to the widening of the passband, the filter rejection level will still be higher than that of a four-pole filter. In this way, the passband performance is significantly enhanced due to the wider bandwidth and a reasonable level of filter rejection is maintained.
As shown in FIG. 1A, the four electrical cavities of a four-pole filter will each result in a peak in the filter's return loss. As shown in FIG. 1B, the five electrical cavities of a five-pole filter will also each result in a peak in the filter's return loss. Finally, as shown in FIG. 1C, the five-pole filter will provide more insertion loss (5 POLE IL in FIG. 1C) (i.e. more rejection) than the four-pole filter (4 POLE IL in FIG. 1C).
Microwave multiplexing network filter performance is particularly important in satellite applications since an increase in the insertion loss of the channel filters in the microwave multiplexing network results in a reduction of Effective Isotropic Radiated Power (EIRP) emitted by the satellite and accordingly a reduction in the amount of radio frequency (RF) transmission power that is converted to thermal dissipation. Insertion loss also limits the transmission of spectral regrowth from the power amplifiers that drive the filters.
Conventional design techniques achieve increased filter rejection by increasing the order of the filter, for example from 4-poles to 5-poles. However, in order to do this, extra resonators are added to realize an additional pole. This approach typically increases the weight and size of the multiplexer which is a significant drawback for extremely weight sensitive satellite applications. Accordingly, prior art microwave filters and multiplexer design processes typically involve optimization of physical cavity structures for a particular channel such that the same filter order is maintained.